The invention generally relates to the field of power electronics. More particularly, the invention relates to a pulse-width modulated (PWM) rectifier wherein the reactive and active powers are controlled independent of one another for improved performance. The PWM rectifier has particular utility in a current source inverter (CSI) based drive for controlling one or more high power alternating current (a.c.) induction motors.
CSI-based a.c. motor drives are increasingly used in high power (e.g., 1,000xcx9c10,000 hp) applications. See, for instance, P. M. Espelage and J. M. Nowak, xe2x80x9cSymmetrical GTO Current Source Inverter for Wide Speed Range Control of 2300 to 4160 volt, 350 to 7000hp, Induction Motorsxe2x80x9d, IEEE IAS Annual Meeting, pp 302-307, 1988. Compared with voltage source inverter fed drives, the CSI drive features simple structure, reliable short circuit protection, four quadrant operation capability and nearly sinusoidal output voltage and current waveforms. In addition, the symmetrical gate turn-off thyristor (GTO) switching devices typically used in CSI drives can be easily connected in series, which makes the CSI drive particularly suitable for implementation at medium/high voltage levels such as 4160 Volts and up. Further details concerning the benefits of the CSI drive can be found in F. DeWinter and B. Wu, xe2x80x9cMedium Voltage Motor Harmonic Heating, Torques and Voltage Stress When Applied on VFDsxe2x80x9d, IEEE 43rd PCIC Conference, pp 131-139, 1996.
In many industrial applications, it is often necessary to control multiple motors in some manner. In these cases, it will be more economical to drive all motors by a single drive system rather than implementing individual drive/motor systems. To date, however, the CSI drive has typically been applied to single-drive/single-motor applications.
The CSI drive is not problem-free. In the CSI drive with a single a.c. induction motor, there exists a resonance mode due to the parallel connection of the output filter capacitor and the motor. This makes it difficult to stabilize the system if the drive operates at a frequency which is close to the resonant frequency. Further details concerning this problem can be found in the following two references, both of which are incorporated herein in their entirety: B. Wu, F. DeWinter, xe2x80x9cElimination of Harmonic Resonance in High Power GTO-CSI Induction Motor Drivesxe2x80x9d, IEEE PESC Conf. pp 1011-1015, 1994; and R. Itoh, xe2x80x9cStability of Induction Motor Drive Controlled by Current-source Inverterxe2x80x9d, IEE Proc. Vol. 136, Pt. B, No. 2, pp 83-88, 1989. The situation becomes even worse when the motor is unloaded since the inverter output current in this case is minimal whereas the resonant current flowing between the capacitor and the motor magnetizing inductance is substantial.
A similar resonance problem also exists when a PWM rectifier is employed in the drive to provide direct current to the CSI from a power source. In this case, a resonance mode exists between an input a.c. filter capacitor of the rectifier and the system impedance of the line voltage source. If the resonance frequency is close to a characteristic harmonic of the rectifier an oscillation will occur, which makes the stability of the PWM rectifier sensitive to the system impedance. Unfortunately it is difficult to measure the system impedance accurately, which complicates the design of a compensating filter. In addition, even when the resonance frequency is not close to any characteristic harmonic of the rectifier, undesired oscillations will also occur during transient states. See additionally, N. R. Zargari, G. Joos, and P. D. Ziogas, xe2x80x9cInput Filter Design for PWM Current-Source Rectifiersxe2x80x9d, IEEE Trans. on Ind. Appl., vol. 30, No. 6, pp 1573-1579, 1994.
When the PWM rectifier is used to provide direct current to the CSI it can sometimes be difficult to tune the control compensators of the PWM rectifier. This will be better understood by reference to FIG. 11 which shows a block diagram of a typical control scheme for a PWM rectifier 100 with unity power factor control which, in conjunction with a smoothing d.c. link inductor Ldc, provides direct current for CSI 110. The rectifier comprises two control loops: a power factor control loop 112 and a d.c. link current control loop 114. In the power factor control loop 112, the phase angle between the source voltage {right arrow over (v)}s and the source current {right arrow over (i)}s is detected by a phase detector 116 and sent to a p.i. proportional, integral) compensator 118 which controls the phase angle xcex1 of the rectifier output current in order to ensure a unity power factor on the voltage source 22. The d.c. link current idc is controlled by adjusting the modulation index M of the rectifier 100 by another p.i. compensator 120 so as to minimize the error between a requested d.c. link current i*dc (this signal is provided by CSI controller) and the actual or measured value of the d.c. link current ixe2x80x2dc. The source voltage {right arrow over (v)}s is detected to provide the synchronizing signal for the rectifier 100. This control scheme is not entirely satisfactory because the phase angle control loop 112 effects not only the power factor but also the d.c. link current. Similarly, the modulation index control loop 114 effects the power factor. Consequently, the coupling between these control loops 112 and 114 can make it difficult to tune the p.i. compensators 118 and 120. Another drawback of this control scheme is that the rectifier 100 maybe saturated under some operating conditions due to the power factor compensation.
In a CSI drive with multiple motors, there are two major technical challenges which must be overcome to make such a drive practical. First, the motors connected to the inverter may have different sizes, which may produce multiple resonant modes. The effect of these and other resonant modes on drive stability should be minimized, and the drive should be able to operate steadily over a full speed range. Second, the inverter output voltage should be kept constant both in steady and transient states for a given output frequency. In other words, the inverter output voltage should be stiff, not affected by changes in multiple motor loads. Otherwise an interaction between the motors and inverter will occur when one or more motors are loaded or unloaded, making the system unstable. A solution to these problems is described herein.
Furthermore, when the PWM rectifier is used to provide d.c. current to the CSI, the problem of tuning the control compensators is present. The invention seeks to overcome this problem.
The general utility of the invention(s) described herein relate to improved CSI-based motor drives. However, those skilled in the art will understand that the various aspects of the invention may be employed more generally in the field of power electronics.
Generally speaking, the invention provides a rectifier which has independent power factor and d.c. link current control loops. This is accomplished by separately controlling the active power and the reactive power of the rectifier.
According to one aspect of the invention a rectifier is provided which comprises a switching bridge for converting alternating current into direct current. The bridge features a line side and a load side. The bridge has m input filter capacitors, each connected at a terminal thereof, to the line side. These terminals are used to connect an m-phase a.c. power supply (m greater than =1) having a per phase system inductance to the bridge. A current smoothing inductor is connected to the load side of the bridge and enables a load to be connected thereto. A switching pattern generator controls the switches in the switching bridge based on a reference output current. A first control loop is provided for determining an active portion of the reference output current based on a desired power level of the load and an m-phase voltage at said terminals. A second control loop is provided for determining a reactive portion of the reference output current based on the m-phase voltage at said terminals. The reactive portion of the current is selected so as to obtain a desired per phase power factor, such as unity, on the power supply.
In a three phase (i.e., m=3) embodiment of such a rectifier is disclosed herein. The illustrated embodiment employs a transformation block for transforming the 3-phase voltage at the capacitor terminals into a two-phase voltage in a rotating reference frame. A low pass filter is connected to the transformation block. The output of this filter provides a fundamental portion of the voltage at the capacitor terminals in the rotating reference frame. A rectifier reference current generator is connected to the filter for determining the active portion of the output current as follows:       i          p      ,      d        =            P      ·              v                  if          ,          d                                    v                  if          ,          d                2            +              v                  if          ,          q                2                  i          p      ,      q        =            P      ·              v                  if          ,          q                                    v                  if          ,          d                2            +              v                  if          ,          q                2            
where ip,d, ip,q are the components of the active portion of the output current in the rotating reference frame, P is the desired power of the load, and vif,d vif,q are components in the rotating reference frame of fundamental portion of the voltage at the capacitor terminals.
The reactive portion of the output current is determined as follows:
icomp,d=2xcfx80xc2x7fxc2x7Cixc2x7vif,q and icomp,q=2xcfx80xc2x7fxc2x7Cixc2x7vif,d
where icomp,d, icomp,q are the components of the reactive portion of the output current in the rotating reference frame, f is the fundamental frequency of the power supply, and Ci is the capacitance of each capacitor.
In the illustrated embodiment the load is an inverter and the desired load power is determined as a sum of first and second components. The first component is a calculated power applied to the inverter. This preferably computed by multiplying the input voltage of the inverter by a measured value of the rectifier output current. The second component is determined by a compensator, such as a p.i. compensator, which seeks to minimize the error between the measured value of the output current and a command signal for the inverter.